1. Field of the Invention
The present invention relates to a charged particle beam system for measuring and inspecting a sample, and more particularly to a system for measuring and inspecting a sample having an insulating film and a pattern fabricated on the insulating film, such as a photomask.
2. Description of the Related Art
The size of a photomask used to manufacture a semiconductor device has been reduced with high density packaging of a pattern. With this tendency, a system for inspecting and measuring a photomask needs to be more accurate. A charged particle beam system represented by a scanning electron microscope (SEM) is mainly used to inspect and measure a photomask.
The photomask is mainly made of quartz that is an insulator. The photomask typically has a front surface on which an integrated circuit pattern is fabricated. The integrated circuit pattern is made of chrome and drawn by an electron beam lithography system or the like. The chrome pattern is covered with a resist pattern (photosensitive agent) drawn by the electron beam lithography system or the like.
For example, when a critical dimension scanning electron microscope is used to measure a photomask, a back surface of the photomask may be charged depending on a pattern manufacturing process, handling of the photomask, or the condition of storage of the photomask due to exposure of quartz (that is an insulator) to the outside of the photomask from the back surface of the photomask.
A charge accumulated on a sample may cause a shift of the focal point of a scanning electron microscope used to measure the sample or cause a magnification fluctuation of the scanning electron microscope. This may cause a measurement error.
JP-A-2001-52642 discloses a scanning electron microscope including a plurality of electrometers and a sample chamber. The electrometers are provided in the sample chamber and measure an electric potential on a front surface of a sample. Based on the measured electric potential, the scanning electron microscope performs a feedback process on an acceleration voltage applied to emit an electron beam and on a retarding voltage.
In addition, WO03-007330 (corresponding to U.S. Pat. No. 6,946,656) discloses a scanning electron microscope that measures an electric potential distribution on the surface of a sample and performs a feedback process on a retarding voltage and the like during a process for transporting the sample.
In addition, U.S. Pat. No. 6,507,474 discloses that an electrometer is used to measure charges accumulated on a target sample such as a mask and an ionizer is used to neutralize the charges.